wgkey: new package

This is a replacement for the key-related parts
of the wireguard-go wgcfg package.

This is almost a straight copy/paste from the wgcfg package.
I have slightly changed some of the exported functions and types
to avoid stutter, added and tweaked some comments,
and removed some now-unused code.

To avoid having wireguard-go depend on this new package,
wgcfg will keep its key types.

We translate into and out of those types at the last minute.
These few remaining uses will be eliminated alongside
the rest of the wgcfg package.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

control/controlclient/netmap.go

@@ -17,6 +17,7 @@ import (
 	"inet.af/netaddr"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/wgkey"
 	"tailscale.com/wgengine/filter"
 )
 
@@ -24,7 +25,7 @@ type NetworkMap struct {
 	// Core networking
 
 	NodeKey    tailcfg.NodeKey
-	PrivateKey wgcfg.PrivateKey
+	PrivateKey wgkey.Private
 	Expiry     time.Time
 	// Name is the DNS name assigned to this node.
 	Name          string
@@ -241,7 +242,7 @@ const EndpointDiscoSuffix = ".disco.tailscale:12345"
 func (nm *NetworkMap) WGCfg(logf logger.Logf, flags WGConfigFlags) (*wgcfg.Config, error) {
 	cfg := &wgcfg.Config{
 		Name:       "tailscale",
-		PrivateKey: nm.PrivateKey,
+		PrivateKey: wgcfg.PrivateKey(nm.PrivateKey),
 		Addresses:  nm.Addresses,
 		ListenPort: nm.LocalPort,
 		Peers:      make([]wgcfg.Peer, 0, len(nm.Peers)),